[sdiy] Audio taper pots

[Donald Tillman]

On Aug 24, 2012, at 10:53 AM, Tim Ressel wrote:

> How important is it to use audio taper pots for audio attenuators? I have a mixer design that needs linear pots for panning. Can I use linear pots for the attenuators? It would mean stocking just one pot, and at 1000 minimum quantity I'd like to avoid needed 2 pots.


Generally audio taper pots are best, for the same reason that we measure sound level in dB.

But it's your design, you get to decide what taper you want.  Then you get to decide which audio taper variation you want (there's more than one audio taper).  Then you get to decide exactly what value of load resistor you'll use and how it will alter that taper.  And you get to put it all in perspective with respect to the goals of your device, how you want the musician to interact with it, and how you want it to feel, which sections of the pot rotation could benefit from coarser or finer adjustment, and so forth.

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Besides loading down the pots, I'd like to point out that there are other ways to get audio-like curves from linear pots.

You can place the linear pot in the feedback loop of an opamp.  That is:

   source through a resistor to the pot CW terminal
   opamp output to the pot CCW terminal
   pot wiper terminal to the opamp negative input
   opamp positive input grounded

(I've omitted numerous details and issues for the sake of brevity.)  

I can't remember who, but long ago some hifi manufacturer used this approach and advertised it as a volume control with a curve that felt more natural.  It's not crazy, as Baxendall tone controls work the same way.

Another approach was used in the preamp of the Ovation Breadwinner/Deacon guitar.  This was the first commercial electric guitar with a built in preamp.  It used a *dual* linear pot, wired up like this: 

   input -> linear pot -> preamp -> linear pot -> output

The linear pots in series create a parabolic curve, which is interesting.  But it's also cool because turning the pot down simultaneously decreases the noise level and increases the headroom.  A downside is that the pot is mechanically more difficult to spin with your pinky.

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Just some food for thought.

See, I solved your problem.  Now all you need to do is prototype a dozen variations and try them out.

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com